Identifying patients who require emergent cardiac catheterization is a daily challenge, but the ECG is an invaluable tool for this. Beyond finding ST elevation in two anatomic leads, other high-risk ECG changes represent an occlusion of an epicardial coronary artery that also requires reperfusion. Learning to recognize these patterns is essential to prevent infarction of significant area of ventricle myocardium.
aVR ST Elevation
The aVR lead was developed to evaluate the basal segment of the septum, and ST elevation is seen in two high-risk situations. aVR elevation occurring along with anterior ST elevation represents a LAD occlusion that carries a higher risk of mortality. Diffuse ST depression, on the other hand, means the aVR elevation is reciprocal, and develops from subendocardial ischemia over a large territory of the ventricle. This can occur with left main insufficiency, proximal LAD insufficiency, or severe multi-vessel coronary artery disease. The changes can develop with ACS or severe supply-demand mismatch such as hemorrhage, tachycardia, or hypotension. Even if the ECG pattern is a result of ACS, the vessels are not acutely occluded so medical treatment is reasonable. Emergent catheterization lab would be indicated if that patient does not respond to maximal therapy.
Spontaneous reperfusion of a LAD occlusion with resolution of chest pain can sometimes generate anterior biphasic or deeply inverted symmetric T waves, with preservation of the R wave voltages, which are known as Wellens' waves. The troponins may or may not become positive, depending on the length of time the LAD was occluded. Mimics of Wellens' waves may occur in left ventricular hypertrophy, severe hypertension, or benign T wave inversion. A diagnosis requires the appropriate clinical scenario. A regional wall motion abnormality would have a good positive predictive value. Provocative testing may be disastrous, as case reports have shown, because these patients have a significant LAD lesion.
Posterior Wall MI
Infarction of the posterior wall is usually associated with occlusions affecting the lateral or inferior walls but can occur in isolation. The posterior wall of the heart is not seen directly by any lead on a standard ECG, which causes posterior MIs to be missed frequently. It is also not electrically silent as many describe it. Posterior wall ischemia is seen as reciprocal changes and manifests as horizontal flat ST depression in V1-V4 with prominent wide R waves and upright T waves, and r/S ratio >1 in V2. These are analogs of anterior lead ST elevation, Q waves, and inverted T waves. Distinguishing a posterior MI from its confounder subendocardial ischemia is challenging. Demand ischemia is more likely than localized posterior occlusion when the ST depression occurs more laterally (V4-V6), the patient is older or has coronary artery disease, and has reasons for severe supply-demand mismatch (tachycardia, anemia, hypotension). A posterior ECG with leads V7-9 can directly view the electrical vector of posterior area and help with the diagnosis if you suspect posterior MI. (Leads are placed on the left posterior thorax at the posterior axillary, midscapular, and paraspinal lines. A guide for placement can be found at http://bit.ly/1UkR8ld.) ST elevation of 0.5 mm in just one of these leads should be considered to represent an occlusion, and should prompt activation of the cardiac catheterization lab.
Anterolateral Wall MI
Occlusion of branches from the LAD can present with non-classical, noncontiguous ST elevation. The first diagonal branch (D1) or ramus intermedius (RI) can be a large vessel that perfuses large parts of the anterolateral myocardium. They are often a target for coronary artery bypass grafting because of their size. Occlusion of these vessels generates ST elevation in aVL and V2, along with upright T waves and inferior ST depression with inverted T waves. These are not considered anatomic continuous leads, but that is mainly because of our simplified visualization of the heart in the thorax. This represents occlusion that requires reperfusion therapy.
de Winter Waves
Total or subtotal occlusion of the proximal LAD can manifest as upsloping ST segment depression after the J-point in the anterior leads V1-V4 with tall positive symmetric T waves. Two forms of this pattern seem to be reported in the literature: one that occurs with complete occlusion and remains stable until reperfusion and a second form that is dynamic and is seen with subtotal occlusion. Ultimately, it does not matter. These patients require immediate reperfusion. The cause of this ECG pattern remains debatable, but likely it is attributable to a K-ATP cardiac channel mutation.
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